Communication systems take many forms. In general, the purpose of a communication system is to transmit information-bearing signals from a source, located at one point, to a user destination, located at another point some distance away. A communication system generally consists of three basic components: transmitter, channel, and receiver. The transmitter has the function of processing the message signal into a form suitable for transmission over the channel. This processing of the message signal is typically referred to as modulation. The function of the channel is to provide a physical connection between the transmitter output and the receiver input. The function of the receiver is to process the received signal so as to produce an estimate of the original message signal. This processing of the received signal is referred to as demodulation.
Two types of two-way communication channels exist, namely, point-to point channels and point-to-multipoint channels. Examples of point-to-point channels include wirelines (e.g., local telephone transmission), microwave links, and optical fibers. In contrast, point-to-multipoint channels provide a capability where many receiving stations may be reached simultaneously from a single transmitter (e.g., cellular radio telephone communication systems). These point-to-multipoint systems are also termed Multiple Access Systems (MAS).
To transmit a message signal (either analog or digital) over a band-pass communication channel, the message signal must be manipulated into a form suitable for efficient transmission over the channel. Modification of the message signal is achieved by means of a process termed modulation. This process involves varying some parameter of a carrier wave in accordance with the message signal in such a way that the spectrum of the modulated wave matches the assigned channel bandwidth. Correspondingly, the receiver is required to recreate the original message signal from a degraded version of the transmitted signal after propagation through the channel. The recreation is accomplished by using a process known as demodulation, which is the inverse of the modulation process used in the transmitter.
In addition to providing efficient transmission, there are other reasons for performing modulation. In particular, the use of modulation permits multiplexing, that is, the simultaneous transmission of signals from several message sources over a common channel. Also, modulation may be used to convert the message signal into a form less susceptible to noise and interference.
For multiplexed communication systems, the system typically consists of many remote units (i.e., subscriber units) which require active service over a communication channel for a short or discrete portion of the communication channel resource rather than continuous use of the resources on a communication channel. Therefore, communication systems have been designed to incorporate the characteristic of communicating between relatively few central communication sites and many remote units for brief intervals on the same communication channel. These systems are termed multiple access communication systems.
One such type of multiple access communication systems is a cellular radiotelephone system. Typical cellular radiotelephone systems have a comparatively large number of radio channels available which, further, can be effectively multiplied by reuse of the radio channels in a geographical area, such as the metropolitan area of a large city like Chicago, by dividing the radio coverage area into smaller coverage areas called "cells" using relatively low power transmitters and coverage restricted receivers. Such cellular systems are further described in U.S. Pat. Nos. 3,906,166 and 4,268,722. The limited coverage area enables the radio channels used in one cell to be reused in another cell geographically separated according to a predetermined plan, such as a four cell channel/frequency reuse pattern shown and described in U.S. Pat. No. 4,128,740. In this four cell channel reuse pattern, each cell is assigned a subset of the available radio channels and reuse of the radio channels is accomplished by repeating the pattern through out a geographical area.
A cellular system typically utilizes a pair of radio frequencies for each radio channel in each cell. Each cell is assigned at least one paging/access channel and several voice channels. The paging/access channel is dedicated to controlling the operation of the subscriber units by means of data messages transmitted to and received from the subscriber units. Control functions performed include paging selected subscriber units, receiving requests for service from subscriber units, instructing subscriber units to tune to a voice channel where a conversation may take place, and enabling subscriber registration by which process the subscriber units identify themselves to the system. The data message and radio channel specifications for U.S. cellular radiotelephone systems are set forth in Electronic Industries Association (EIA) Standard EIA-553 (hereinafter "EIA-553"). Copies of EIA-553 may be obtained from the Engineering Department of the Electronic Industries Association at 2001 Pennsylvania Street, N.W., Washington, D.C. U.S.A. 20006. EIA-553 is also known as Advanced Mobile Phone Service (AMPS). Additionally, a similar, but enhanced signaling protocol specification has been proposed to Telecommunication Industry Association (TIA) TR-45 engineering committee, known as Narrow Advanced Mobile Phone Service (NAMPS). Copies of this proposal may also be obtained from the Engineering Department of the Electronic Industries Association at 2001 Pennsylvania Street, N.W., Washington, D.C. U.S.A. 20006.
A cellular system can grow by adding more voice channels to existing cells, subdividing existing cells into smaller cells and adding new cells until the call switching capacity of the cellular system control terminal is consumed, or until the traffic density limit inherent in the particular channel reuse pattern is reached. At that point, additional switching capacity is required for further growth, or a different channel reuse pattern must be adopted. Alternatively, additional communication systems can be overlaid onto portions of the previous communication system coverage area to add new services and features for subscribers while accommodating the pre-existing population of subscribers. These additional communication systems can also relieve some of the communication traffic congestion by serving subscriber units which are adapted for use in the previous communication system and the overlaid additional communication systems. However, such enhanced subscriber units optimally will need to be able to operate in the previous as well as the added communication systems. Accordingly, there is a need for an improved method and apparatus for determining a serving communication system where two or more radiotelephone communication systems conjunctively cover the same geographical area to allow subscriber units capable of operating in either communication system to preferentially select one communication system over the other communication system when both are available to the subscriber unit at a particular location.